1. Field of the Invention
This invention relates to lithiated oxides with a well-layered crystal structure for use as cathode materials, and methods of manufacturing such materials.
2. Background Art
Rechargeable lithium batteries are of commercial interest due to their high energy and power density, and their long cycle life. Lithium ion batteries (i.e. batteries which do not use metallic lithium as the anode) have replaced lithium batteries since extended cycling using lithium metal as the anode is problematic. The majority of rechargeable lithium ion batteries use anode materials, which do not contain lithium metal, for example carbon or tin containing materials. This has resulted in the requirement that cathodes must contain lithium which can be extracted during first charge, as well as cycle well over several hundreds of charge-discharge cycles. Prior to the introduction of lithium ion batteries, all materials which cycled well, including sulfides, were candidates for cathodes. Since the introduction of Li-ion batteries the choice of materials has in practice drastically reduced.
Most interesting as cathodes for rechargeable lithium ion batteries are lithium transition metal oxides such as spinel Li1+xMn2xe2x88x92xO4 (and its modifications) and LiCoO2. Spinel Li1+xMn2xe2x88x92xO4 is low cost and does not contain hazardous materials. However, its applications are limited since the capacity of materials which cycle well is only around 115 mAh/g. Additionally, the capacity retention during cycling at elevated temperatures (if cycled at 55xc2x0 C. for example) is not sufficient. LiCoO2 cycles well, and has a capacity of approximately 140 mAh/g, however Co is toxic and expensive. Promising alternatives to Li1+xMn2xe2x88x92xO4 and LiCoO2 are LiNiO2-based materials. Nickel is less toxic and less costly than Co. Furthermore, in cells LiNiO2 has a larger reversible capacity than LiCoO2.
LiNiO2 and LiCoO2 are layered materials having the alpha-NaFeO2 type structure (space group R-3m). Layered materials are of interest for cathodes since a layered structure provides for the fast diffusion of lithium. In this structure, layers of transition metal octahedrally surrounded by oxygen (leading to MO2 sheets) are separated by lithium cations in the lithium layers. The formula can be written as LiMO2. In this document the formula will often be written as Li[M]O2, or as {LiM}[M]O2, where [M] stands for all cations residing in the transition metal layers, and {M} stands for non-lithium cations in the lithium layers. In many prior art LiNiO2-based materials, the lithium layers are partially filled by transition metal ions. An example is nickel rich LiNiO2 which can be written as {Li0.9Ni0.1}[Ni]O2. Especially promising for cathode applications are well-layered materials, i.e. materials which have little or no transition metal M located on lithium sites. Such well ordered structures provide for fast lithium intercalation and de-intercalation.
It is difficult to prepare LiNiO2 with an acceptable capacity retention during extended cycling. LiNiO2 materials also generally suffer from some irreversible capacity loss in the first cycle, i.e. less lithium can be reinserted during first discharge than was extracted during first charge. A large irreversible capacity is undesirable for practical applications. Good capacity retention and small irreversible capacity correlate with a well-layered crystal structure. An ideal well-layered crystal structure has a large c:a ratio and a small amount of transition metals mislocated on lithium sites. However, in practice, preparation of samples with only a small amount of Ni in the Li layer sites is difficult. The amount of Ni on lithium sites can be estimated from Rietveld refinements of X-ray diffraction data. Alternatively, Dahn et al. in Solid State Ionics 44 (1990) 87 defined an R-factor which sensitively correlates with the concentration of Ni on lithium sites. R is defined as the ratio of integrated intensities of the 101, 006 and 102 peaks of the diffraction pattern of the layered material having the R-3m structure. Many prior art disclosures suggest ways to prepare LiNiO2 with a small amount of misplaced Ni on lithium sites, i.e. they try to prepare LiNiO2 materials with a small value R, but these methods have not completely solved the problem, or are not economically feasible.
Another basic problem of LiNiO2-based materials is that they become very reactive if overcharged, i.e. charged to voltages where significantly more than around 60% of the nickel is oxidized from the 3+ to the 4+ state. In large cells the overcharged cathode decomposes slowly, generating more heat than the cell can release to the environment. This accelerates the decomposition reaction ultimately leading to thermal runaway with explosion, ignition or venting of the battery. In practice large batteries cannot use LiNiO2 as cathodes because they tend to go into thermal runaway, and are therefore unsafe.
Dahn et al. in Solid State Ionics 69 (1994) 265 showed that charged LiNiO2 is hazardous since Li1xe2x88x92xNiO2 contains the very reactive tetravalent Ni ion. The cathode tends to react to form a rocksalt type LixNi1xe2x88x92xO as discussed in Arai et al. in Solid State Ionics 109 (1998) 295. In the rocksalt structure Ni has a more preferred lower average valence state. The reaction is accompanied by a release of oxygen which can react with the electrolyte.
Doping LiNiO2 with less reactive or non-reactive materials can lead to materials where the reactive Ni4+ is diluted. Substitution of Ni ions with other cations has also been shown to improve the electrochemical performance in some cases. For example, U.S. Pat. No. 5,750,288 (Rayovac) issued May 12, 1998, describes the modification of LiNiO2 by substituting up to 30% of the Ni by a non-transition metal element from the group Al, Ga, Sn and Zn. Substitution has been shown to improve the safety of LiNiO2-type materials to some degree by doping with Al by Ohzuku et al in J. Electrochem. Soc. 142 (1995) 4033, and by doping with Mg1/2Ti1/2 by Gao et al in Electrochemical and Solid State Letters 1 (1998) 117.
Substitution of nickel with a fraction of cobalt (e.g. Li[Ni1xe2x88x92xCox]O2 where x is about 0.2 to 0.3) can lead to a material with good electrochemical properties. Such materials are described for example by Delmas et al. in J. Power Sources 43/44 (1993) 595. However, the safety problems associated with LiNiO2 are not completely solved, as shown by Paulsen and Dahn in Abstract 43, Proceedings of the 195th Meeting of the Electrochemical Society, Seattle, May 2-6, 1999. Furthermore the substitution of nickel by cobalt increases the cost of the material relative to LiNiO2.
The use of manganese as a dopant in LiNiO2-based compounds has been anticipated to provide certain advantages. Since manganese is cheap and non-hazardous, a large Mn content in an LiNiO2 based cathode would be desired not only for safety considerations but also for price.
Substitution of Ni in LiNiO2 by manganese was described by Dahn et al. in Solid State Tonics 57 (1992) 311. In that report it was shown that LiNiO2 can be substituted with manganese leading to Li[Ni1xe2x88x92xMnx]O2 with a maximum substitution limit of x being approximately 0.5. Where x greater than 0.5, the materials were not monophase but a phase mixture containing Li2MnO3, and LiyNi1xe2x88x92yO with the rocksalt structure. However it was reported that LiNiO2 with large amounts of manganese, especially Li[Ni1/2Mn1/2]O2, did not cycle well.
U.S. Pat. No. 5,264,201 (Dahn et al.) issued Nov. 23, 1993, described a method for making LiNi1xe2x88x92yMyO2 where M may be Co, Fe, Cr, Ti, Mn or V and y is less than about 0.2 (with the exception that y is less than about 0.5 when M is Co). The method was intended to provide a material which was substantially free of lithium hydroxide or lithium carbonate, and which had improved cycling capacity over unsubstituted LiNiO2. However this disclosure stated that the improved cycling capacity was only maintained when up to 20% of the Ni is replaced by Co, Fe, Cr, Ti, Mn or V (or up to 50% for Co), and did not describe the preparation of compounds with higher amounts of substitution.
U.S. Pat. No. 5,370,948 (Matsushita) issued Dec. 6, 1994, described a method to produce LiNi1xe2x88x92xMnxO2 where x is between 0.05 and 0.45. Compositions where Mn is substituted for Ni up to 50% were described, but the crystal structures of the compounds were not well-layered. This was evidenced by the X-ray diffraction data presented in the disclosure, which show only a single diffraction peak in the region 63-66 degrees two theta, described as the 110 peak. Layered rhombohedral compounds with c/a ratios greater than 1 show two peaks in this region (hexagonal 108 and 110 peaks). It is furthermore clear from the X-ray diffraction data presented for the higher amounts of Mn substitution (eg 40% Mn) that the compounds became less layered for increasing amounts of Mn substitution. There was also evidence for the formation of a second phase (probably Li2MnO3) in the data for 40% Mn substitution.
U.S. Pat. No. 5,626,635 (Matsushita) issued May 6, 1997, described a process to produce LiNi1xe2x88x92yMyO2 compounds where M is either Co or Mn. Where M is Mn, y was preferred to be equal to or less than 0.3. According to the disclosure, when more than 30% of Ni was substituted by Mn, crystalline growth became difficult and the materials cycled poorly when used as cathodes in secondary lithium cells. The process for making the materials recommended restricting the temperature range between 600 and 800 degrees. According to the disclosure, if materials containing Mn substituted for Ni were heated above 800 degrees, Mn mislocated in the Li sites of the crystal structure causing disorder and deterioration of the discharge capacity and cycle life.
U.S. Pat. No. 5,393,622 (Matsushita) issued Feb. 28, 1995, described a method to prepare LiyNi1xe2x88x92xMxO2 with 0 less than xxe2x89xa60.3 and 1xe2x89xa6yxe2x89xa61.3. The method was a multi-step solid state reaction using a Li salt, Mn oxide or carbonate, and Ni carbonate or hydroxide. Materials were prepared with varying lithium content. It was shown that additional Li (y greater than 1) could be inserted into layered Mn doped LiNiO2. However it was observed that not more than 0.3 mol Mn could be introduced to the crystal structure. When x was higher than 0.3, a lower crystallinity was observed, and when x was 0.4 a Mn spinel peak appeared in the X-ray diffraction data, showing formation of a second phase. The materials described for x up to 0.3 were layered phases having c/a (=chex/(240.5ahex)) ratios of approximately 1.010, and chex axes generally xe2x80x9cbeing in the range 14.15 less than c less than 14.24 xc3x85xe2x80x9d, i.e. not as well-layered as may be achieved by the present invention described in the present disclosure.
U.S. Pat. No. 6,045,771 (Fuji Chemical Industry Co.) issued Apr. 4, 2000, described a process to prepare Liyxe2x88x92x1Ni1xe2x88x92x2MxO2, where M represents one element selected from the group consisting of Al, Fe, Co, Mn and Mg, x=x1+x2, 0 less than x1xe2x89xa60.2, 0xe2x89xa6x2xe2x89xa60.5, 0 less than xxe2x89xa60.5, and 0.9xe2x89xa6yxe2x89xa61.3. The compounds disclosed included LiN1xe2x88x92xMnxO2 where x was up to 0.4, however the increasing amount of Mn resulted in an increasing capacity fade.
Japanese Patents JP3047693 and JP3042128 (Matsushita) also described batteries comprising LiNi1xe2x88x92xMnxO2 materials made by specialized processing routes, but once again the capacity decreased markedly when the Mn content was increased above 0.3, therefore compositions with 0 less than x less than 0.3 were preferred.
European Patent Application EP 0 918 041 (Fuji Chemical Industry Co.) described materials LiyNi1xe2x88x92xCox1Mx2O2 wherein 0.9xe2x89xa6yxe2x89xa61.3, 0 less than x xe2x89xa60.5, 0 less than x1 less than 0.5, x1+x2=x, and when M is Mn 0 less than x2xe2x89xa60.3. Only one material containing Mn was described, where x2=0.05. U.S. Pat. No. 6,040,090 (Sanyo Electric Co.) issued Mar. 21, 2000, and European Patent Application EP 0 944 125, also described lithium-metal compound oxides containing combinations of Ni, Co and Mn. The compounds disclosed all had at least 49% Ni (as percentage of total Ni+Co+Mn), and a maximum of 40% Mn (as percentage of total Ni+Co+Mn).
Combinations of Ni, Co and Mn were also described in European Patent Application EP 0 782 206 (Japan Storage Battery Company), however a significant decrease in capacity of the batteries was observed when the amount of Mn was increased above 30% (as percentage of total Ni+Co+Mn), therefore the preferred compositions contained Mn at less than 30%. The addition of Al was also considered desirable to improve the thermal safety characteristics.
In summary, although the potential benefits of substituting large amounts of Mn into LiNiO2 materials have been frequently discussed in the prior art, materials where large amounts of Mn have been substituted have not been demonstrated to provide acceptable electrochemical properties. This can generally be related to a poor layer character of the crystal structure or to the trend that manganese rich LiNiO2 tend to phase separate to multi-phase mixtures.
It is an object of the present invention to provide lithiated oxide materials with a well layered crystal structure which reduce or overcome at least some of the abovementioned problems, or which at least provide the public with a useful alternative.
Other objects of the present invention may become apparent from the following description which is given by way of example only.
According to one aspect of the present invention there is provided a single phase cathodic material of composition Li[LixCoyA1xe2x88x92xxe2x88x92y]O2 where A=[Mn2Ni1xe2x88x92z], 0.4xe2x89xa6zxe2x89xa60.65, 0 less than xxe2x89xa60.16 and 0.1xe2x89xa6yxe2x89xa60.3, and wherein the additional x lithium is included in the transition metal layers of the structure. In a preferred form, the material is a single-phase well-layered R-3m crystal structure having a c/a ratio greater than 1.012 (where this ratio is defined as chex/241/2ahex).
In one preferred form 0.05xe2x89xa6xxe2x89xa60.10, y is substantially equal to 0.16, and z is substantially equal to 0.50.
According to a further aspect of the present invention there is provided a method of manufacture of a material of composition Li[LixCoyA1xe2x88x92xxe2x88x92y]O2where A=[MnzNi1xe2x88x92z], 0.4xe2x89xa6zxe2x89xa60.65, 0xe2x89xa6xxe2x89xa60.16 and 0.1xe2x89xa6yxe2x89xa60.3, the material is a single-phase well-layered R-3m crystal structure having a c/a ratio  greater than 1.012 (where this ratio is defined as chex/241/2ahex), and the additional x lithium is included in the transition metal layers of the structure, the method including the steps of: preparing a precursor with mixed metal cations including Ni, Mn and Co, mixing the said precursor with stoichiometric amounts of a Li source, and reacting the resulting mixture at an elevated temperature. In one preferred embodiment, it is contemplated that the precursor be heated at a temperature in the range substantially 500-1000xc2x0 C. in an oxygen containing atmosphere. It is also contemplated that such a temperature may be in the range substantially 900-1000xc2x0 C. Preferably for at least 12 hours.
Preferably, the said precursor may be a mixed metal hydroxide.
Preferably, the mixed metal hydroxide may be prepared by co-precipitation from a solution containing Ni, Mn and Co.
In an alternative form the said precursor may be a mixed metal oxide M3O4, where M is a combination of Ni, Mn and Co.
Preferably, the M3O4 may be prepared from a mixed metal hydroxide.
In an alternative form the said precursor may be a mixed metal oxide MO, where M is a combination of Ni, Mn and Co.
Preferably, the mixed metal oxide MO may be prepared from a mixed metal hydroxide.
In an alternative form the mixed metal precursor may be a mixed metal oxide LixMO2 where M is a combination of Ni, Mn and Co and wherein x is substantially 1. Preferably, the lithium source may be Li2CO3.
According to a further aspect of the present invention there is provided a method of producing a material of composition Li[LixCoyA1xe2x88x92xxe2x88x92y]O2 where A=[MnzNi1xe2x88x92z], 0.4xe2x89xa6zxe2x89xa60.65, 0 less than xxe2x89xa60.16 and 0.1xe2x89xa6yxe2x89xa60.3, the material is a single-phase well-layered R-3m crystal structure having a c/a ratio greater than 1.012 (where this ratio is defined as chex/241/2ahex), and the additional x lithium is included in the transition metal layers of the structure, the method including the steps of: mixing powders of metal oxides, the metal oxides including Ni, Mn, Co and lithium, ball milling the mixture to produce a precursor oxide, and heating the precursor oxide.
In one preferred form the metal oxides may be selected such that the total lithium and oxygen content has substantially the stoichiometry required in the final lithiated oxide material.
In a further preferred form heating the precursor oxide or reacting the mixture may be at a temperature in the range substantially 500-1000xc2x0 C. in an oxygen containing atmosphere.
Preferably, the temperature may be in the range substantially 900-1000xc2x0 C. Preferably for at least 12 hours.
According to a further aspect of the present invention there is provided a method of manufacture of a lithiated oxide material having a layered crystal structure substantially as herein described and with reference to any one of the accompanying examples of the invention and/or figures.
According to a further aspect of the present invention there is provided a lithiated oxide material having a layered crystal structure substantially as herein described and with reference to any one of the accompanying examples of the invention and/or figures. According to a further aspect of the present invention there is provided a cathode for use in a secondary lithium ion electrochemical cell, said cathode including as active material a lithiated oxide material as herein before described.
According to a further aspect of the present invention there is provided a secondary lithium ion electrochemical cell including a lithium intercalation anode, a suitable non-aqueous electrolyte including a lithium salt, a cathode including as active material a lithiated oxide material substantially as herein described, and a separator between the anode and the cathode.
According to a further aspect of the present invention there is provided use of a lithiated oxide material as herein before described in the manufacture of a cathode and/or a secondary lithium ion electrochemical cell.
Other aspects of the present invention may become apparent from the following description which is given by way of example only and with reference to the accompanying figures.